The human spine is composed of seven cervical, twelve thoracic, five lumbar vertebrae, the sacrum and the coccyx. Vertebrae increase in size starting from the cervical spine through the bottom of the lumbar spine and are generally shaped as oval cylinders with concave upper and lower surfaces/endplates. Mobile segments of the cervical, thoracic and lumbar spine are joined by paired left and right arthrodial joints posteriorly, intervertebral discs anteriorly and circumferential stabilizing ligaments. The spine serves the dual role of providing skeletal support for the trunk, maintaining appropriate skull position and protecting the spinal cord and spinal nerves from trauma or compression.
Appropriate spinal alignment is characterized by balance in the coronal (left-right) and sagittal (anterior-posterior) planes. Among other concepts, balance involves the centering of a plumb line dropped from the neck or skull over the pelvic midline to ensure that excessive soft tissue stresses are not required to achieve or maintain good posture. Radiographic conventions are used to define balance. In the sagittal plane, the C7 vertebra is said to be in balance if a plumb line dropped from the center of the vertebra intersects the superior/posterior corner of the S1 vertebra. In the coronal (side to side) plane, the spine is said to be in balance if a plumb line dropped from the midpoint/spinous process of C7 intersects the midpoint of the S1 vertebra.
Radiographic criteria defining acceptable sagittal balance are sometimes defined at the top of the thoracic and bottom of the lumbar spine, because of variations among individuals in the extent of natural thoracic kyphosis (apex posterior concavity) and lumbar lordosis (apex anterior convexity). There exists significant variation in terms of not only the degree of kyphosis and lordosis but also in the location/level of the transition between kyphosis and lordosis (inflection area). Still, with regard to the lumbar spine, the lowest disc spaces, L3-L4, L4-L5 and L5-S1 contribute to the greatest extent to the aggregate lordosis of the lumbar spine.
Over three decades ago, Francis Denis proposed a three column concept of spine stability. His widely recognized system divides the spine into anterior, middle and posterior columns. The anterior column is comprised of the anterior longitudinal ligament and the anterior half of the vertebral body and intervertebral disc, the middle column comprises the posterior longitudinal ligament and posterior half of the vertebral body and intervertebral disc and the posterior column comprises the transverse processes, pedicles, laminae, facet joints, ligamentum flavum and posterior spinal ligaments including the supraspinous and interspinous ligaments.
As the spine ages, a predictable sequence of degenerative changes occur. Aging brings a loss of hydration of disc cartilage and derangement of the disc's internal architecture that can contribute to a loss of disc height and a variable degree of circumferential bulging. As a result of our uniquely upright posture, the structural curvature of the spine and gravity, the lowest three intervertebral discs are most frequently affected by degenerative changes at the earliest age and most severely as degenerative changes progress.
Loss of disc height and disc bulging with or without concomitant bulging of adjacent fibrous joint capsule tissues and osteophyte (bone spur) formation may result in irritation or impingement of adjacent neural structures. Another common cause of nerve irritation is a disc herniation characterized by displacement of fragmented disc material toward the spinal canal. Nerve root irritation causes radiating “sciatic” leg pain that typically radiates from the lower back to one or both legs. Some further believe that degenerative or damaged intervertebral discs may be the source of severe low back pain in some patients.
Degenerative changes that affect intervertebral discs may also lead to deformities of the spine in either the sagittal or coronal plane. Curvature in the coronal plane is termed a “scoliosis”; loss of balance in the sagittal plane is termed “positive” imbalance if it results in an anterior shift in posture and “negative” imbalance if it results in a posterior shift.
Degenerative loss of disc height leads to a disproportinate shortening of the anterior and middle columns of the spine relative to the posterior column that may retain its length better in the face of aging. Aging influences sagittal balance in the direction of positive imbalance for two principal reasons. First, the lowest lumbar discs are usually the most lordotic in the healthy spine, and those discs are also among those most frequently affected by degenerative loss of height. Also, loss of lordosis in the lower lumbar spine has the greatest effect on neck and head position because those discs are furthest away and have the longest radius of action.
Spine surgeons are becoming increasingly aware that age-related changes in sagittal spinal balance may be a key factor in the development of back pain and decreased quality of life and mobility. Though further research is required in this area, it is possible that spine surgeons historically underestimated the degree of spinal sagittal malalignment by neglecting the effect of changes in the sacropelvic axis that initially compensate for positive sagittal imbalance. As the spine tilts forward out of sagittal balance, the pelvis may tilt backward (retrovert) to maintain posture. This rotation decreases the anatomic range of motion of the hip joint in extension and it moves the base of the spine posterior to the weight bearing axis of the hip joints.
A variety of surgical techniques have been developed to treat degenerative spine conditions. Most relevant to this invention, spinal fusion procedures are intended to immobilize adjacent vertebrae by means of a surgically created osseous bridge. Spinal fusion is performed for patients whose spine is initially unstable or where removal of bone, joints, disc or ligamentous structures to effect required decompression of neural elements would render the spine unstable. Fusion is also necessary after surgical techniques are used to correct spinal deformities to ensure that the spine will durably retain its corrected position.
Surgical correction of spinal alignment or balance is accomplished using a variety of surgical techniques. Among these, spinal osteotomies, often in concert with the use of intervertebral implants, are the most widely performed. Corrective osteotomies involve either shortening of the posterior column only (e.g. Smith-Petersen osteotomy), or shortening of all three columns of the spine (e.g. pedicle subtraction osteotomy). Selection among osteotomy techniques involves an assessment of the degree of deformity, the amount of correction desired and the spinal levels where the osteotomy is planned.
Intervertebral implants have been developed to assist surgeons in achieving the goals of lumbar spine fusion surgery. Interbody implants may be composed of a variety of biocompatible materials with differing moduli of elasticity, and they may be implanted after a variety of surgical approaches including the anterior, lateral and posterior approach to the spine. Relevant to the instant invention, the posterior approach to the disc space has the benefit of not requiring an additional surgical incision and in general does not endanger the lumbar plexus, the intestines nor the great vessels as a lateral or anterior approach would. However, the posterior insertion of intervertebral implants is limited by the degree to which the neural structures can be mobilized and retracted to make room for passage of the implant. Typically, the eponymous “Kambin's Triangle” bounded by the exiting nerve root laterally, the traversing nerve root medially and the superior border of the caudal vertebra is <15 mm in transverse dimension.
The internal trabecular anatomy of the vertebrae has been studied carefully both by histologic analysis and by radiographic measurement. Consistently, it has been shown that trabecular thickness, density and bone strength are higher in the posterior aspect of the vertebral body, especially in the area near the medial border of the base of the pedicle. This pattern may be expected to be particularly characteristic in the lower lumbar vertebrae where the weight-bearing access of the spine falls preferentially in the middle column, and Wolff's law would suggest that bone would be stronger in this region as a biologic reaction to increased weight-bearing stress.
A multitude of intervertebral implants intended for posterior implantation exist. Though various geometries exist including devices that change shape or expand once they have been inserted into the disc space, several challenges remain:                a) Maximum implant size is limited by the medial-lateral and cephalad-caudal dimension of the implant, especially at the upper part of Kambin's Triangle where the exiting nerve root is most medial. Medial mobilization of the traversing nerve root requires additional surgical time and risk. Through less invasive approaches, surgical visualization of medial or lateral neural structures and may be limited.        b) The maximum degree of lordosis of the implant is constrained as the largest side of the implant is the first part inserted into the disc space, and this largest dimension of the implant must accommodate passage through Kambin's triangle. Especially in weakened, osteoporotic bone, forcible insertion of the largest part of the implant may result in partial endplate fracture that propagates forward along the track of insertion (plowing) resulting in an implant that has compromised structural support.        c) As a result of constraints on the maximum achievable degree of lordosis, surgeons have not been able to rely on interbody devices alone to achieve restoration or augmentation of lordosis. Implants inserted from a PLIF or TLIF approach or technique are especially limited in terms of restoration of lordosis because of the geometric constraints discussed above.        d) Devices that are inserted with smaller dimensions and are expanded once they are inserted into the interbody space carry the potential for mechanical failure of the expansion mechanism either during insertion making expansion impossible or resulting in a potentially loose implant that may be difficult to remove once partially expanded in the intervertebral space. Lack of tactile feedback during implant expansion may result in endplate fracture and implant subsidence. The design of expandable implants are generally require the implant to be solid, without a graft window, and the footprint of the graft becomes an area where interosseous fusion cannot take place.        e) To determine the most appropriate implant size, multiple trial implants if increasing size are generally inserted prior to insertion of the final, retained implant. This process is time consuming, involves multiple forcible passes of instruments adjacent to neural structures and with each passage, it increases the likelihood of damage to the endplate.        
Accordingly, there is a need for an improved intervertebral implant, intended for implantation through a posterior approach, that addresses these drawbacks.